Robert Colvile is a writer and senior comment editor at the Telegraph, who cares more about politics and policy than is probably healthy - for his newest pieces, please see here. He tweets as @rcolvile.

Ed Miliband must win back Labour's missing millions

So which is the real Ed Miliband? The one who prowls around the conference stage, beating the drum for "One Nation" politics? Or the one who rallies the comrades at the TUC's latest rally against "the cuts" (something for which, if the Government were in any less disarray, he'd have been skewered)?

Lacking the ability (or the desire) to peer into Ed's soul, I'll settle for telling him not which he is, but which he needs to be. Or rather, I'll let Peter Kellner do it. For while there's long been anguished discussion within the Tory Party about how it can win back the millions of voters lost since the electoral highpoint of John Major's 1992 campaign, Labour has millions of missing supporters too. And in the new edition of Prospect, Kellner and his team at YouGov work out exactly where they've gone.

In 1997, says Kellner, Tony Blair won 13.5 million votes. In 2010, Gordon Brown got 8.6. Given that the latter figure includes all those supporters who came of age after 1997, that's many millions of people who stopped voting for the party, and who'll be needed if it wants to get back into power. So where did they go?

Well, as of September 2012, four million of Blair's voters have died, and 6.4 million remain loyal (including almost 1.7 million who have re-ratted from the Lib Dems since the formation of the Coalition). But that still leaves three million people – almost a tenth of the likely electorate – who might conceivably be up for grabs. If Ed Miliband can win over even a fraction of them, the path to Downing Street will be wide open.

But here's where things get tricky. What's the difference, Kellner asks, between those who left Labour and those who stayed? Not much, on the surface: loyalists and defectors "live in similar homes, have similar jobs, watch similar TV shows and drive similar cars". What distinguishes them is their politics. A few, some 400,000, left Labour because it was too far to the Right. But the bulk – some 2.6 million – are far more centrist, or even Right-wing. For example, they're four times more likely to read the Mail/Sun/Telegraph/Star/Express than the Mirror or the Guardian (whereas current Labour voters divide evenly).

The differences don't end there. As Kellner says, "just 21 per cent [of defectors] want the government 'to do far more to help the poor'". Many more – 27 per cent – want welfare cut, because "the poor should take more responsibility to for themselves". Most of the defectors – 59% – want Britain out of the EU, and the overwhelming majority – 78% – want net immigration down not from the hundreds to the tens of thousands, but to zero. (Incidentally, the fact that 41% of Labour loyalists also want out of the EU and 67% want "one in, one out" on immigration is yet another example of how far all parties in Westminster are from their own supporters on these issues.)

The voters who abandoned Labour are, in short, those squarely on the middle ground of British politics – the normal, decent, hard-working "strivers" whom all three parties are trying desperately to attract.Kellner argues that this means that any class war message from Labour – "any whiff of the politics of social contest" – or any lunge back to the Left will alienate support rather than attract it. Of course, given that – in the killer question – only 3% of these former Blair supporters believe that Labour "is led by people of real ability", a more basic problem for Ed Miliband might be getting them to listen to him at all.